Tunable liquid crystal lenses are described for example, in commonly-owned PCT patent application WO2009/153764 published Dec. 23, 2009 wherein a tunable liquid crystal lens uses a frequency dependent material (as defined therein) to shape an electric field and a corresponding refractive index gradient within a liquid crystal cell, suitable for creating a lens.
In the article titled “Liquid crystal lens with focus movable in focal plane” by Ye et al. published in Optics Communications, 259 (2006) 710-722, there is disclosed a liquid crystal lens in which the optical axis of the lens is movable by controlling the relative amplitudes of control signals applied to each quadrant of a four segment hole-patterned electrode. The lens arrangement uses the segmented electrode below a planar electrode, both electrodes which are fed a control signal to define the electric field as a function of relative amplitudes of fixed frequency control signals applied to the upper planar electrode and the lower segmented electrodes with respect to a bottom planar electrode. Experimental results demonstrating the effect of varying the relative amplitudes, show a shifting or change in the optical axis of the liquid crystal lens.
Tunable liquid crystal (LC) optical devices, such as lenses, operate with a uniform electric or magnetic control field, most use a spatially modulated field. Employing electric fields, there are a few prior art techniques used to spatially modulate the electric field. Spatially inhomogenous dielectric layers have been used to attenuate the electric field to provide a desired spatial profile. Electrodes have been spherically shaped to provide a desired electric field spatial profile. Another approach to spatially modulating the electric field is to use a planar electrode whose impedance properties are such that the voltage drop over the electrode as AC drive current is fed to the electrode leads to a spatially modulated electric field.
As shown in FIG. 1, one type of conventional LC cell is built by sandwiching the liquid crystal 102 between two substrates 104,106, each of which is first coated with (by) a transparent electrode 108, 110, which can be a layer of material such as indium tin oxide (ITO), and then coated by polymer layers 112 (typically polyimide) which are rubbed in a predetermined direction to cause alignment of LC molecules in a ground state, namely in the absence of a controlling electric field. The application of voltage to two ITO layers creates a uniform electric field and correspondingly uniform LC molecular reorientation which correspondingly provides a uniform refractive index distribution across the LC layer. In such a device, the index of refraction is different in a direction lengthwise with respect to the molecules than in a direction transverse to the molecules.
FIG. 2 illustrates a prior art LC cell configuration, in which a hole patterned electrode ring 204 of low resistivity surrounding a disk-shaped zone 205 of high resistivity material is used to generate an electric field gradient. This geometry has the advantages of being very thin (which is a key requirement, e.g., in cell phone applications) and of using only two electrodes (and thus one voltage control drive signal). Unfortunately, it is difficult to produce the required thickness of high resistivity material with high optical transparency, as well it is hard to produce an LC cell with good uniformity, and the manufacturing process typically has a low yield. Different lenses will have slightly different electrode resistances and this, coupled with the fact that the required control is also very dependent on the precise cell thickness, means that each individual lens needs to be calibrated separately. Also, the minimum diameter of a modal lens is limited to about 2 mm—below this size the required resistivity of the ITO layer exceeds some 10 MΩ/sq. Finally, such (so called “modal control”) lenses must always be either positive or negative. It is not possible to switch between a diverging and converging lens.
FIG. 3 illustrates another prior art LC cell configuration with electric field gradient generation. Three distinct electrodes 304, 305, 307 (two of them in the inter-hole pattern formed on the same plane), and two voltages V1 and V2 are used with an additional distinct weakly conductive layer (WCL) 306. The role of the external hole patterned electrode 304 (with voltage V1 applied thereto) is to create a lens-like electric field profile, while the role of the central disk-shaped electrode 305 (with voltage V2 applied thereto) is to (avoid) reduce disclinations and to control the value of the gradient (e.g., to erase the lens). The role of the WCL 306 is to soften the profile created by V1 and to allow for the reduction of the overall thickness of the lens. Unfortunately, the complex patterning of the top electrode, the necessity of using two distinct voltages and a separate WCL are difficult to manufacture and inhibit the practical use of this approach. For example, the use of this approach to build a polarization independent lens would require the use of six to seven thick glass elements, which is a difficult task.